how do I find FOS by Bishop method in terms of varying Cohesion ,Gama and Phi value??

i want to find FOs eqn having cohesion ,gama and phi value as variable.

i wrote this code.

upo = sum((p.*l) + (W.*cost*tanp) - (u.*l*tanp));

downo = sum(W.*sint);

FSo = upo./downo; %%for initial guess%%

FSt = 1.2*FSo; %%to maintain thelogic in while loop%%

tol=0.001;

while abs(FSt-FSo)>tol

FSo = FSt;

Nu = W +(T.*sina)-(((p.*l.*sint) - (u.*l.*tanp.*sint))./FSo);

Nb = cost + (tanp.*sint)./FSo;

N = Nu./Nb;

up = sum((p*l*R) + (N.*tanp*R) - (u.*l*tanp*R))+((FSo.*T*(R.*cosaw)));

down = sum(W.*xavg);

FSt = up./down %FOS after reinforcement%%

end